The invention relates generally to temperature measurement apparatus and, more particularly, to thermocouple devices for use in continuously sensing the temperature of molten metals, specifically molten steel.
It is desirable in continuous steel casting operations to monitor the temperature of the molten metal in the various metallurgical vessels, such as in the ladle and the tundish. Significant improvements have been made in recent years with respect to the refractory components which ensheathe the thermocouple assembly so as to prolong service life when immersed in a molten metal. Such improved refractory materials for protective thermocouple sheaths are disclosed and claimed in U.S. Pat. No. 4,721,533, Phillippi et al., owned by the assignee of the present invention. Even though these improved refractory materials and components have contributed to increase the service lift of immersion thermocouples, high temperature premature failures still occur due to high temperature chemical reactions occurring along the thermocouple lead wires. In addition, localized condensation of certain compound results in a so-called emf drift of the thermocouple wherein temperature readings become inaccurate. It has been observed in a submerged tundish application of a continuous caster that complete wire fractures or open circuit failures have occurred in the loop area of the thermocouple lead wires after only 10 to 50 hours of operation. A 100-hour service life is desired, as a minimum for this type of metallurgical application.
In prior thermocouple designs, a pair of platinumrhodium alloy lead wires are threaded within a double bore insulator leaving an exposed loop length of wire extending outside one end of the insulator. The double bore insulator and contained lead wires are housed within an alumina sheath having a closed end that encloses the exposed end of lead wire. The alumina sheath is then placed in a closed-end molybdenum sheath. The molybdenum sheath, in turn, is housed in an outer refractory sheath which protects the thermocouple assembly from the molten metal.
In such prior design, the spaces inside the thermocouple probe are packed with high-purity alumina powder. In addition to the annuli between the molybdenum and alumina sheaths, and the alumina sheath and double-bore insulator, the alumina powder is packed in the space above the inner alumina sheath, nearly to the full length of the steel extension pipe which is connected to the molybdenum sheath Furthermore, air flow into the probe head is restricted to seepage around the connector and through slits in the welded pipe joints. Thus, the internal atmosphere within the thermocouple assembly approximated a closed system. A few milligrams of carbon-containing contaminants. Thus, a minute amount, on the order of milligrams or less of carbon containing contaminants introduced, inadvertently or otherwise, inside the probe during assembly has an opportunity to create a reducing atmosphere at the base of the probe. This creates a dangerous environment for the platinum alloy loop, particularly if silica is also present. Silica is present as a naturally occurring trace contaminant in most manufactured ceramics, as well as in the atmosphere of most ceramic manufacturing plants. There is no theoretical barrier preventing the following reaction to proceed until one of the reactants--carbon, silica or platinum--is completely consumed. The reaction set forth below is believed to be responsible for the premature separation of platinum alloy thermocouple wires: SiO.sub.2 +2C+XPt=Pt.sub.X Si+2CO. The inter-metallic Pt.sub.X Si can have a melting point several hundred degrees lower than steel.
Due to the extremely high temperatures encountered during service, on the order of about 2800.degree.-2900.degree. F., it is theorized that very minute amounts of carbon, possibly present in the alumina powder and in normal handling, along with trace amounts of silica on or in the alumina sheath contribute to chemical attack of the platinum-rhodium thermocouple lead wire. It has been observed by microscopic examination that wire separation has commonly occurred at the grain boundaries of the exposed loop section of wire, wherein silicon containing compounds have been subsequently detected by electron probe and other analytical methods. It is further theorized that the molybdenum along the bore of the molybdenum sheath reacts with oxygen and forms a very mobile gas at molten steel temperatures which subsequently condenses as a solid on the exposed thermocouple lead wires causing subsequent emf drift problems.
It is, therefore, an object of the present invention to provide an improved thermocouple assembly which overcomes the dual problems of rapid open circuit failure and emf drift commonly found in prior devices of this type.
The present invention provides a thermocouple assembly which limits the strength and duration of a reducing atmosphere around the loop region by effectively allowing reducing gases, such as carbon monoxide, to dissipate from the loop region whereby the service life of the platinum-rhodium lead wires is greatly extended. Still further, the present invention provides a thermocouple assembly in which the migration or diffusion of oxygen towards the bore of the molybdenum sheath is extremely slow, so as to minimize the oxidation of molybdenum and the subsequent gaseous transport and deposition which is theorized to occur.
Still further, the invention provides a thermocouple assembly which includes a sacrificial getter element in the loop region therein which further protects the thermocouple lead wires from chemical attack. The loop is further protected by recessing in the end of the double bore insulator and capping the end of the double bore insulator with a noble metal or ceramic.